The invention relates to time-of-flight mass spectrometry (TOFMS), and more particularly to a system that limits the detection of undesired ions.
In TOFMS, ions of different mass travel at different velocities along a flight path, creating a mass spectrum of ions. As a result of the different travel velocities, the ions in the mass spectrum strike a detector at the end of the flight path with a time distribution that enables the mass spectrum to be determined.
In TOFMS, it is often desirable to remove ions having a certain mass from the mass spectrum before the ions contact a detector. For example, it may be desirable to remove ions of a certain mass from the mass spectrum because the quantity of those ions is orders of magnitude higher than the ions of interest, or because the removed ions are from a known material that is of no interest to the analysis.
One known technique for removing undesired ions from a mass spectrum of ions involves ion deflection. Ion deflection is a technique in which a field of electrical potential is applied to a portion of a flight path at the moment that the target ions pass by the portion of the flight path that includes the electrical potential. The applied electrical potential causes the target ions to be deflected from the flight path such that the deflected ions do not impact a detector.
FIG. 1 is a depiction of a system 10 for implementing TOFMS with ion deflection. The key components of the system include an ion generator 12, a flight tube 14, a deflector 16, and a detector 18. The ion generator generates ion pulses that travel down the flight tube toward the detector. By knowing the time of flight for a particular mass of ions, the deflector can be momentarily activated to deflect a target mass of ions from the original flight path and out of the path of the detector. It should be noted that neither FIG. 1 nor the figures that follow are drawn to scale.
One known technique for accomplishing ion deflection utilizes a plate deflector and another known technique utilizes an interleaved-comb deflector. FIG. 2 is a depiction of the plates 22 of a plate deflector and example flight paths 24 of deflected ions 20. With regard to the plate deflector, two plates are placed along a flight tube (not shown), with both of the plates being generally parallel to each other and to the flight tube. The two plates are spaced apart so that ions pass between the plates as the ions travel through the flight tube. An electrical potential is applied between the deflector plates just as undesired ions pass between the plates, causing the target ions to be deflected from their original flight path and into a flight path that does not include the detector.
FIG. 3 is a depiction of the wires 28 of an interleaved-comb deflector and example flight paths 30 of deflected ions 20. With regard to the interleaved-comb deflector, an array of parallel wires is placed generally perpendicular to the original flight path. Equal but opposite-polarity voltages are applied to alternating wires at the time that target ions pass the parallel wires. The applied potential creates an electrical field that deflects the ions from their original flight path as shown in the example of FIG. 3. Other known interleaved-comb deflectors may utilize a series of interleaved parallel plates instead of wires to deflect ions.
Whether utilizing the plate deflector technique or the interleaved-comb deflector technique, the distance with which ions are deflected from their original flight path is a function of both the field created by the deflector and the length of the flight path that remains after the deflector. For example, referring to FIG. 4, a strong deflection field and a large distance (L) between the deflector 34 and the detector 36 will create a large deflection (D) of the actual flight path 38 from the original flight path 40. The angle of deflection (xcex8) will depend on the strength of the deflection field, as well as other factors.
In order to prevent undesired ions from contacting the detector, the undesired ions must be deflected onto a flight path that does not include the detector. If the electrical field created by the deflector is too low, the undesired ions will not be deflected far enough from the original flight path to avoid striking the detector. However, there are concerns related to employing high strength deflection fields. Higher fields have a greater susceptibility to transmission or coupling of voltage pulses into the detector. Moreover, the equipment required for creating the higher strength fields (e.g., power supplies) tends to be more expensive. As a result, there is a tension between the need to adequately deflect unwanted ions and the desire to employ small electrical fields. In view of the tension between providing adequate ion deflection while minimizing the size of the applied electrical field, what is needed is a method and system that can deflect undesired ions out of a spectrum of ions with a relatively low electrical field created by the deflector.
A time-of-flight mass spectrometer includes a deflector and a filter assembly that is located along a linear or non-linear flight path between the deflector and an ion detector. The filter assembly passes incoming ions along the flight path to the detector when the ions approach the filter assembly along their original flight path, and the filter assembly occludes incoming ions from the flight path when the ions have been deflected from their original flight path by the deflector. In an embodiment, the filter assembly includes filtering plates that are aligned such that the major surfaces of the filtering plates are parallel to the original flight path of the ions. In order to remove ions of a particular mass from a mass spectrum of ions, target ions are deflected from their original flight path to cause the target ions to impact the filtering plates, while the ions that are not deflected from their original flight path pass between the filtering plates for measurement by the detector.
A preferred time-of-flight mass spectrometer includes an ion generator, a flight tube, the deflector, the detector, and the filter assembly. The ion generator, the flight tube, the deflector, and the detector, are all conventional devices that are used in time-of-flight mass spectrometers. The deflector is preferably either a plate deflector or an interleaved-comb deflector.
The filter assembly, which is the focus of the invention, is located along the flight path of the ions between the deflector and the detector. In an embodiment, the filter assembly includes a series of parallel filtering plates that are aligned such that the major surfaces of the filtering plates are in parallel with the original flight path of the ions. The filtering plates are maintained at the same voltage throughout the analysis process. The filtering plates are preferably passive elements that are not electrically manipulated, which is in contrast to the deflector which includes active elements that are electrically manipulated to deflect the ions.
The filtering plates that form the filter assembly function to reduce the acceptance angle for ion detection, where the acceptance angle is defined as the largest angle of flight relative to the filtering plates which will still allow passage of ions through the plates. Ions that do not enter the filter assembly at an angle that is parallel, or nearly parallel, with the major surfaces of the filtering plates will likely contact a filtering plate and be occluded from the flight path. With the filtering plates in place, a smaller angle of deflection is sufficient to prevent deflected ions from being detected by the detector.
The time-of-flight mass spectrometer equipped with the filter assembly of the invention can be utilized to accomplish mass-specific filtering. In order to accomplish mass-specific filtering the deflector is activated to deflect ions of a particular mass when the ions of that particular mass pass the deflector. The ions that are deflected from the original flight path no longer travel in parallel with the major surfaces of the filtering plates and therefore are occluded from the deflector. The ions of the desired masses are not deflected by the deflector and because they continue to travel along their original flight path, they pass between the parallel filtering plates and are easily detected by the detector.
With the filtering plates in place, target ions can be occluded from the detector by deflecting ions from the original linear or non-linear flight path at a smaller angle, and consequently with a smaller electrical field, than would be required to completely bypass the detector. In contrast, prior art systems require that ions be deflected with a large enough electrical field that the ions bypass the detector.